1. Field of the Invention
This invention relates to a cathode ray tube, and more particularly to a cathode ray tube in which a beam of electrons is deflected by an electromagnetic deflector and a static deflector.
2. Description of the Related Art
As shown in FIG. 6 of the accompanying drawings, a cathode ray tube is a glass bulb having a panel 1 and a funnel 2. A beam of electrons is produced by an electron 5 gun 4 located in the neck 3 of the funnel 2 and deflected by a deflection yoke 7 near a cone 6 of the funnel 2. The beam of electrons 5 is then focused onto a fluorescent screen layer 8 inside the panel 1 and is scanned so as to reproduce an image.
A television receiver is required to be compact and thin. However, the television receiver is also required to have a large display screen. It is therefore essential to make the cathode ray tube as thin as possible. One approach for this purpose is to enlarge a maximum deflection angle of the electron beam. This approach will be described with reference to FIG. 7. As described above, an electron gun 4 produces a beam of electrons, a direction of which is changed by a deflection yoke 7 while the electron beam passes through a magnetic field generated by the yoke 7. An angle by which the electron beam is redirected is called the "angle of deflection". When the electron beam is scanned at a periphery of a fluorescent screen, it has a maximum deflection angle. The length of the cathode ray tube depends upon the maximum deflection angle of the electron beam. Specifically, when the display screen has a height 2S, the electron beam 5 from the electron gun 4 is deflected at a deflection point 0 with an angle .theta.. It is assumed that the electron beam has a deflection angle .theta..sub.0 (maximum deflection angle) at the periphery of the display screen. An overall length of the cathode ray tube, F, is expressed as follows: EQU F=T+L+M+G EQU L=S/tan .theta..sub.0
where L represents a length between the deflection point 0 and the display screen , M a length between the deflection point 0 and the forward edge of the electron gun 4, G a length of the electron gun, and T a thickness of the panel. According to this formula, L can be reduced by enlarging the maximum deflection angle .theta..sub.0, which means a reduction in the length of the cathode ray tube. Table 1 shows a relationship between the deflection angles and the entire length F of a 37-inch cathode ray tube as an example.
TABLE 1 ______________________________________ (Max. deflec. angle .THETA..sub.0) .times. 2 Whole length F ______________________________________ 90.degree. 1090 mm 110.degree. 810 mm 130.degree. 590 mm 150.degree. 440 mm ______________________________________
where T+M+G=150 mm.
The larger the maximum deflection angle, the shorter the cathode ray tube as a whole. However, it is necessary to raise the level of energy applied to the deflection yoke and intensify the electric field to be generated when the deflection angle is made as large as possible while keeping the electron beam at a predetermined energy level. For this purpose, an electromagnetic deflector having a high output level should be used, which means a possible increase in the size of the television receiver and in power consumption.
Further when deflection angle is large, the electron beam will be radiated onto the fluorescent screen 8 with a large incident angle .PHI., thereby causing distortion of a reproduced image in the peripheral region of the display screen.
Japanese Patent Laid-Open Publication Sho 64-82435 (1989) exemplifies a method for reducing an incident angle of the electron beam by deflecting the electron beam electromagnetically once and deflecting it statically twice.
With the foregoing example, the electron beam has not only a high acceleration voltage but also a high energy level. Therefore, the magnetic field should be strong enough to cope with such an electron beam. In addition, a voltage for static deflection should be high enough. Application of the high voltage requires that both the electromagnetic deflector and the static deflectors should be large. A power supply for these deflectors would inevitably become large too. Such large apparatuses would consume a large amount of power.